1. Field of the Invention
The present inventions relate to a novel poly-.epsilon.-caprolactone resin having an excellent hydrolysis stability and a method for retarding the hydrolysis rate of poly-.epsilon.-caprolactone.
2. Description of the Related Art
A poly-.epsilon.-caprolactone resin has been widely used in a variety of fields because of advantages of high crystallinity, high hardness in the room temperature and relatively low melting point (i.e., 50.degree. to 60.degree. C.) of crystal parts. For example, this resin is used as a toggle operating body (Japanese unexamined patent publication No. 60-240,492) and a plastic material (Japanese unexamined patent publication No. 61-42,679). Furthermore, the resin is widely used as an optimum material in the field where a model of a part of the human body is prepared and the model is fitted to the human body, such as for a medical gypsum material (Japanese unexamined patent publication No. 58-81,042), a splint material, a face mask for shielding from radiation, or a modelling material for a periwig (Japanese Unexamined Patent Application No. 59-70,021).
A poly-.epsilon.-caprolactone resin having a number average molecular weight of 10,000 to 200,000 is generally used in the above-mentioned fields, and this resin is prepared by subjecting .epsilon.-caprolactone to ring-opening polymerization in the presence of an appropriate catalyst and an initiator having an active hydrogen atom.
However, the poly-.epsilon.-caprolactone resin prepared according to this process has a problem in that, when the resin is stored for a long time, the molecular weight is reduced by hydrolysis and the mechanical strengths are abruptly lost. This problem is commonly observed in polyester resins having an ester linkage, and is especially conspicuous in the poly-.epsilon.-caprolactone resin.
When an .epsilon.-caprolactone monomer is polymerized, a trace amount of water contained in the monomer acts as an initiator for ring-opening polymerization. The obtained poly-.epsilon.-caprolactone resin has the following structure: ##STR2## wherein n is an integer of 85 to 1,800 on the average. Namely, the residual carboxyl group is inevitably present at the terminal of the molecular chain. If water in the .epsilon.-caprolactone monomer is completely removed to 0% and an initiator such as ethylene glycol is used, a carboxyl group will not be formed at the terminal of the molecule. However, it is practically impossible to remove water completely to 0%.
By the carboxyl group present at the terminal of the molecule, the hydrolysis common to linear polyesters is further promoted catalytically, and the polymerization degree is reduced with the lapse of polymerization time.
Accordingly, when a pellet or flake of the resin is stored or in the market or an end article such as a medical gypsum material, splint material, a face mask or a modelling material for a periwig is stored or in the market in the form of a plate or rod, the degree of polymerization of the resin is reduced, and sometimes the deterioration by hydrolysis is advanced to such an extent that the resin cannot be put into practical use. Especially in summer, the deterioration by hydrolysis is more conspicuously promoted than in the other seasons.
For preventing the above-mentioned deterioration, various attempts have been made. For example, delicate maintenance control such as for low-temperature storage has ordinarily been carried out.
Furthermore, there has been adopted a process in which a carbodiimide represented by the general formula: EQU R--N.dbd.C.dbd.N).sub.n R'
wherein R and R' may be the same or different, and are hydrocarbon groups having 1 to 24 carbon atoms (e.g., cyclohexyl and isopropyl), or an epoxy resin is added as a stabilizer against the hydrolysis in a small amount to a poly-.epsilon.-caprolactone resin to control the hydrolysis (Japanese unexamined patent publication No. 61-241,354 filed by the same assignee). The carbodiimide includes, for example, N,N'- bis(2,6-diisopropylcyclohexyl)carbodiimide and N,N'-dicyclohexyl-carbodiimide.
However, the carbodiimide has a relatively high skin-irritating property and use of the resin composition as a medical gypsum material, a radiation-shielding face mask or a medical splint material, which is in direct contact with the skin, is limited.
Furthermore, since the carbodiimide has a high reactivity with water or other polymerization initiators and discoloration by exposure to heat is conspicuous, the carbodiimide cannot be added during the step of polymerization of the monomer.
Accordingly, the carbodiimide is post-blended with the resin and, therefore, blending equipment becomes necessary. Moreover, deterioration is caused by heating and mechanical mixing for the post blending of the carbodiimide.